1. Field of the Invention
The present invention relates to surveillance system sensors, and, more particularly, to an arrangement for detecting tampering with a surveillance system sensor.
2. Description of the Related Art
Surveillance systems, also known as security systems, are known to include various types of sensors for monitoring a building structure or an area of space for certain phenomena or events, such as a breach of security. For example, the security sensors may include door sensors for detecting the opening and closing of a door, window sensors for detecting the opening and closing of a window, or passive infrared (PIR) sensors for detecting motion or the presence of a living being.
Door/window sensors are known to be in the form of a pushbutton that is mounted to the door/window frame and is held in a depressed state by the door or window when the door or window is in a closed position. When opening, the door or window moves away from the pushbutton, thereby releasing the pushbutton from the depressed state. A controller monitors the state of the pushbutton, and may issue an alarm signal if the door or window is opened without authorization. Door/window sensors may also be optically based, wherein the reception of an optical signal is either disrupted or initiated upon the opening of the window or door. As another alternative, door/window sensors may be magnetically based. More specifically, a magnet may be placed near the outer perimeter of the door or window, and a reed switch may be mounted in the frame of the door or window such that the reed switch is adjacent to the magnet when the door/window is closed. In operation, the reed switch may continuously or periodically monitor the presence of the magnetic field. If the door/window is opened, the reed switch detects the absence of the magnetic field and informs a controller.
For detecting motion, PIR sensors may be mounted to a wall about eight feet off the floor. The PIR sensor may sense the presence of a warm body within its field of view by detecting the infrared radiation produced by the body.
A problem with many of these types of security sensors is that it is possible for an intruder to defeat the sensor it by various techniques that include removing the sensor from the surface to which it is mounted. As one example, a PIR sensor may be removed from its mounting and the PIR sensor may then be redirected in a direction such that any intruders will not be within the sensor's field of view. As another example, it is possible to defeat an optical door/window sensor by removing the optical sensor from its mounting while replicating the optical signal such that reception of the signal is not interrupted while the door or window is opened.
What is needed in the art is a security sensor that cannot be easily defeated by techniques that involve removing the sensor from the surface to which it is mounted.